1. Technical Field
The present invention relates to a light emitting element, a light emitting device, a display device, and an electronic device using the light emitting element.
2. Related Art
Organic electroluminescence elements (so-called organic EL elements) are light emitting elements each having a structure in which at least one light emitting organic layer is disposed as a light emitting layer between an anode and a cathode. In such a light emitting element, by applying an electric field between the cathode and the anode, electrons are injected into the light emitting layer from the cathode side and also holes are injected thereinto from the anode side, so that the electrons and the holes are recombined in the light emitting layer to generate excitons, and when the excitons return to the ground state, the energy of the excitons is emitted as light.
In such a light emitting element, a hole injecting layer and a hole transporting layer are generally provided between the anode and the light emitting layer in order to increase the injection properties and the transporting properties of the holes (e.g., Japanese Patent No. 3654909).
Moreover, in such a light emitting element, the electrons from the cathode side (light emitting layer side) are blocked by the hole transporting layer by adjusting the size of HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) of the hole transporting layer to confine the electrons and the hole in the light emitting layer, thereby increasing the light emitting efficiency.
However, in the former light emitting elements, the hole transporting layer has not been able to sufficiently block the electrons from the cathode side, which has caused a problem that the hole transporting layer or the hole injecting layer has been deteriorated due to prolonged use by the electrons which have entered and passed through the hole transporting layer. Such a problem becomes remarkable with an increase in a drive voltage (with an increase in a current density), because the function of blocking the electrons of the hole transporting layer decreases due to the band bending effect. Therefore, in a high-intensity light emitting element requiring a high-density current, it has been difficult to achieve an extension of the lifetime.
In order to increase the electron blocking effect, there is a possibility of using a material with a large energy gap between HOMO and LUMO for the hole transporting layer. However, there has been a problem that since the types of materials usable as the hole transporting layer are limited, the materials have been difficult to actually use.